This invention relates generally to communications of digital signals over bandlimited, unequalized, noisy transmission channels at high data rates and, more particularly to the use of microcomputers such as personal computers and the like to accomplish multi-frequency modulation and demodulation for transmission of a multitude of frequencies over a single channel.
In the modern world, computer to computer is quickly becoming the fundamental link for message and data traffic; i.e., information exchange and propagation. In some cases the link is one of many in an extensive network; in others, it may be represented by a single point-to-point communications link. The physical medium constituting the link for transmitting the data may be wire or optical fiber. Or, it may be a microwave or satellite link utilizing radio frequency propagation. (Satellites are also being developed for broadcasting digital audio and digital television signals.) In all cases, the actual signal used to carry the data bits must be a properly modulated analog signal with sufficient energy and of the appropriate frequency to propagate effectively through the channel. Regardless of the form of the link, the objective is to transmit information at a high rate with a low rate of errors from the transmitter to the receiver.
Prior art modulation methods for bandpass channels utilize amplitude and/or phase modulation to carry signal information on a carrier wave in a channel frequency band. When the information source is a finite alphabet, numbers, data, etc., or quantized analog sources such as digitized audio, television, or facsimile, then only a finite number of signal states are required to represent, or code the source.
Digital data is frequently communicated using pulse code modulation techniques at baseband, i.e., in the frequency spectrum from 0 that is to some maximum, or upper frequency limit. If a bandpass transmission is required, i.e., is transmission in a frequency band between an upper frequency and a lower frequency, single side-band (SSB) modulation into and demodulation from the desired frequency band may be employed. For example, polar voltage pulses at 48K bits/sec are filtered to attenuate frequencies above 36 KHz and SSB modulated onto a 100 KHz carrier to fit into the bandpass channel from 60-104 KHz.
Frequency modulation techniques are also employed for digital data. Typically, these involve sending one of two, frequency shift keying (FSK), or one of M (M-ary FSK), frequencies spaced across the available frequency band and may be used in applications where bandwidth efficiency is unimportant as they operate at less than one bit per Hz of available bandwidth.
Alternatively, digital information may be encoded onto a basically analog carrier frequency, centered in the available frequency band, using phase modulation (PSK), differential phase modulation (DPSK), or a combination of phase and amplitude modulation known as quadrature amplitude modulation (QAM). For example, transmission of data over telephone lines designed to carry analog voice frequency signals (VF) is restricted to a frequency band of about 300 to 3500 Hz. Digital modulation of an 1800 Hz carrier frequency using QPSK (2 bits per symbol) or 16-QAM (4 bits per symbol) utilizing a modulator-demodulator device (modem) provides data communication at 4800 and 9600 bits/sec, respectively, when transmitting at a rate of 2400 symbols per second. The unprocessed modulator carrier signal thus provided has a bandwidth substantially greater than the available channel bandwidth so it must be filtered to fit in the available band. These analog filters must be carefully designed so as not to introduce smearing, known as inter-symbol interference (ISI) between adjacent symbol waveforms or the symbols will be decoded in error. A raised cosine filter function is typically employed. Even then, the filtering action of the bandlimited VF channel will introduce ISI due to non-linearities (group delay) in its phase response. These non-linearities are most pronounced at the band edges so that the symbol waveforms must be filtered to the 2400 Hz band from 600 to 3000 Hz before being sent over the channel. The receiving section of the demodulator contains an adaptive filter known as an equalizer in order to remove any residual ISI introduced by the group delay in the 600 to 3000 Hz band. The equalizer must be trained to the particular group delay characteristics of each switched channel connection before any data can be transmitted.
The degree to which the equalization can reduce the ISI on actual circuits limits the number of symbol waveforms that can be distinguished from one another at the receiver and hence limits the number of bits that can be encoded into each baud. A baud is a digitally encoded symbol waveform. In practice it has been found that the combination of ISI and additive noise limit transmission to either two, or under ideal conditions four, bits per baud when transmitting 2400 bauds per second over 2-wire switched telephone circuits.
In order to increase the rate at which data is transmitted, some modems employ multi-frequency modulation (MFM) techniques. MFM utilizes multiple carrier frequencies within the available bandwidth, each frequency independently modulated with digital information in phase and/or amplitude. The frequencies are linearly combined and transmitted as a single digitally encoded waveform, termed a baud, during a finite time interval called the baud interval. U.S. Pat. No. 4,731,816 issued to Dirk Hughes-Hartogs on Mar. 15, 1988 and U.S. Pat. No. 4,601,045 issued to Daniel P. Lubarsky on July 15, 1986 disclose examples of modems employing MFM techniques. Hughes-Hartogs teaches minimizing inter-baud interference by introducing a small guard time between successive bauds during which no signal is sent to prevent received baud overlap. Lubarsky teaches concentrating most of the signal in the center of the baud interval thus causing the signal to taper off to zero near the ends of the baud and minimizing interference between baud intervals by eliminating abrupt changes. If the baud interval is of sufficient duration compared to the guard time or taper time, then the loss of data rate is relatively insignificant.
If the bauds are to be long in duration, then many bits, and consequently many frequencies, may be transmitted in each baud. However, in order to prevent inter-frequency interference, the frequencies must not be spaced too closely. Ideally, the frequencies are made orthogonal over one baud interval. This is known as orthogonal frequency division multiplexing (OFDM). A frequency set will be mutually orthogonal if the frequencies are separated at multiples of the reciprocal of the baud interval. For example, a system employing a rate of 10 bauds per second in the VF band from 300 to 3500 Hz could contain 320 frequencies spaced 10 Hz apart. If each frequency is encoded with 4 bits of information, then 1280 bits will be encoded into each baud and the system will have a throughput rate of 12,800 bits/sec.
The major disadvantage associated with the MFM (OFDM) systems described above is that although they can effectively eliminate inter-baud interference and inter-frequency interference within a baud, they must be demodulated using fully coherent receivers for each frequency in order to obtain the high data rates desired. Since the multitude of frequencies are subject to different and unknown amplitude and phase changes introduced by the transmission channel, these channel properties must be measured during the initiation phase of the communication process and prior to transmission of data. One example of this technique is disclosed by Hughes-Hartogs. Such methods are exceedingly complex and computationally intensive easily rivaling the cost and complexity of adaptive equalization. Furthermore, such OFDM systems will not be effective for direct MFM signalling in bandpass systems such as the 60-104 KHz example described above or in a prototype model of a UHF satellite sound broadcasting system as discussed by Alard et al in "A New System of Sound Broadcasting to Mobile Receivers", presented at the "Centre Commun d'Etude de Telecommunication et Telediffusion" in France and published by the IEEE in 1988.
Differential encoding of the carrier frequencies provides a practical solution to this problem with an attendant 3-db loss of signal-to-noise ratio performance against additive noise. The conventional method to differentially encode information is from baud to baud as is customary in conventional DPSK and as done in the OFDM system disclosed by Alard et al. However, this method produces undesirable results when the baud interval is long, due to channel instability such as that introduced by fading. Further if asynchronous or packet transmissions are utilized there may be a significant reduction of data rate when only two or three bauds are sent since differential encoding in time requires utilizing one baud as a reference.
Frequency differential encoding of multiple carries was utilized in the HF modem disclosed by Gene C. Porter in "Error Distribution and Diversity Performance of a Frequency-Differential PSK HF Modem", IEEE TRANS. ON COMMUNICATION TECHNOLOGY, 16-4, August 1968, pages 567-575. While frequency differential encoding minimized many of the above described problems encountered with OFDM systems, the circuitry required for generation and demodulation of the signals was unduly complicated and did not reliably maintain the necessary orthogonality between the carrier frequencies.